QGridlayout changes height of row

Question

I have a problem with a QGridLayout. One row of my layout contains an element (QProgressbar) that is normaly hidden. When there is some progress to report i call show on it. The problem is that when i call show on the QProgressbar the row above the row containing it will be slightly resized in height (1-3 px). So the whole layout does a little "jump" which looks ugly.

I have given a minimalRowHeight to the row that contains the QProgressbar that is much larger then the height of the QProgressbar but still the height of the row will increase on show().

I have attached a very minimal version of my program that demonstrates the problem. Can anyone give me a hint what is going on there?

Header:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QMainWindow>
#include <QLineEdit>
#include <QtWebKit/QWebView>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);

private:
    QLineEdit* input;
    QWebView *webview;

private slots:
    void slotLoadButton();
};

#endif // MAINWINDOW_H

Source: #include "mainwindow.h"

#include <QProgressBar>
#include <QPushButton>
#include <QGridLayout>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QGridLayout *grid = new QGridLayout;

    input = new QLineEdit;

    QPushButton *loadButton = new QPushButton("load");
    connect(loadButton, SIGNAL(clicked()),
            this, SLOT(slotLoadButton()));

    webview = new QWebView;
    QProgressBar *progress = new QProgressBar;
    progress->setFixedHeight(25);
    progress->hide();

    connect(webview, SIGNAL(loadStarted()),
            progress, SLOT(show()));

    connect(webview, SIGNAL(loadProgress(int)),
            progress, SLOT(setValue(int)));

    connect(webview, SIGNAL(loadFinished(bool)),
            progress, SLOT(hide()));

    grid->addWidget(input, 0, 0);
    grid->addWidget(loadButton, 0, 1);
    grid->addWidget(webview, 1, 0, 1, -1);
    grid->setRowMinimumHeight(2, 35);
    grid->addWidget(progress, 2, 1);

    QWidget* widget = new QWidget;
    widget->setLayout(grid);
    setCentralWidget(widget);
}

void MainWindow::slotLoadButton()
{
    QUrl url = input->text();
    webview->load(url);
}

Answer 1

This looks like a bug in Qt. Try reporting it

This is a workaround:

//grid->addWidget(progress, 2, 1);
QHBoxLayout *l = new QHBoxLayout;
l->addWidget(progress);
QWidget *w = new QWidget;
w->setLayout(l);
grid->addWidget(w, 2, 1);

Answer 2

This is likely caused by the vertical spacing and/or margins of the layout. You should try playing with those properties.